Search

International audience; The combination of aerial images acquired in the visible and near infrared spectral ranges is particularly relevant for agricultural and environmental survey. In unmanned aerial vehicle (UAV) imagery, such a combination can be achieved using a set of several embedded cameras mounted close to each other, followed by an image registration step. However, due to the different nature of source images, usualregistration techniques based on feature point matching are limited when dealing with blended vegetation and bare soil patterns. Here, another approach is proposed based on i mage spatial frequency analysis. This approach, which relies on the Fourier-Mellin transform, has been adapted to homographic registration and distortion issues. It has been successfully tested on various aerial image sets, and has proved to be particularly robust and accurate, providing a registration error below 0.3 pixels in most cases.

International audience; The combination of aerial images acquired in the visible and near infrared spectral ranges is particularly relevant for agricultural and environmental survey. In unmanned aerial vehicle (UAV) imagery, such a combination can be achieved using a set of several embedded cameras mounted close to each other, followed by an image registration step. However, due to the different nature of source images, usualregistration techniques based on feature point matching are limited when dealing with blended vegetation and bare soil patterns. Here, another approach is proposed based on i mage spatial frequency analysis. This approach, which relies on the Fourier-Mellin transform, has been adapted to homographic registration and distortion issues. It has been successfully tested on various aerial image sets, and has proved to be particularly robust and accurate, providing a registration error below 0.3 pixels in most cases.

International audience; Most methods for retrieving foliar content from hyperspectral data are well adapted either to remote-sensing scale, for which each spectral measurement has a spatial resolution ranging from a few dozen centimeters to a few hundred meters, or to leaf scale, for which an integrating sphere is required to collect the spectral data. In this study, we present a method for estimating leaf optical properties from hyperspectral images having a spatial resolution of a few millimeters or centimeters. In presence of a single light source assumed to be directional, it is shown that leaf hyperspectral measurements can be related to the directional hemispherical reflectance simulated by the PROSPECT radiative transfer model using two other parameters. The first one is a multiplicative term that is related to local leaf angle and illumination zenith angle. The second parameter is an additive specular-related term that models BRDF effects.Our model was tested on visible and near infrared hyperspectral images of leaves of various species, that were acquired under laboratory conditions. Introducing these two additional parameters into the inversion scheme leads to improved estimation results of PROSPECT parameters when compared to original PROSPECT. In particular, the RMSE for local chlorophyll content estimation was reduced by 21% (resp. 32%) when tested on leaves placed in horizontal (resp. sloping) position. Furthermore, inverting this model provides interesting information on local leaf angle, which is a crucial parameter in classical remote-sensing.

International audience; In the context of the RHEA European project addressing site-specific weeding on arable crops, a complete aerial image acquisition chain has been developed, providing a geo-referenced multispectral image of a total parcel with 1 cm spatial resolution. Robust automatic ground control point detection and near infrared - visible sensor image registration modules have been implemented and associated with the MicMac photogrammetric software suite to provide a fully automatized mosaicking procedure. This procedure has been successfully applied on various maize and wheat crops for weed patch detection, but could be extended to other precision agriculture applications, especially those requiring a very high spatial resolution.

International audience; A spatialized surface energy balance model was validated over the database acquired in the framework of the ReSeDA program. The benefit of the SEBAL model we considered was to compute wind speed and air temperature using the information contained in the spatial variability of convective fluxes. The multitemporal database allowed performing a validation over cycles of several crops. Problems induced by mixed pixels were reduced using high spatial resolution remote sensing data. We verified the validity of the model basic assumption, i.e. the simultaneous presence of partial areas with very high and very low evaporation rates, and the resulting relation between surface temperature and albedo. Besides, the model provided estimates of wind speed and air temperature close to the field references. The validation of soil heat flux showed the inadequacy of the empirical relationship used through a significant underestimation of the references. The validation of sensible heat flux provided similar results as compared to previous studies that dealt with model validations over databases including numerous situations.; Nous avons validé un modèle de bilan d'énergie de surface en utilisant la base de données ReSeDA. L'intérêt du modèle SEBAL était d'estimer la vitesse du vent et la température de l'air à partir de l'information contenue dans la variabilité spatiale des flux convectifs. La base de données multitemporelle permettait d'effectuer une validation en considérant les cycles de plusieurs cultures. Les problèmes dûs aux pixels mixtes étaient réduits par l'utilisation de données télédétectées à haute résolution spatiale. Nous avons pu vérifier la validité de l'hypothèse de base du modèle, i.e. la présence simultanée de zones très évaporantes ou peu évaporantes, ainsi que la relation entre albedo et température qui en résulte. De plus, le modèle a fourni des estimations de la vitesse du vent et de la température de l'air proche des mesures de référence. La validation du flux de chaleur dans le sol a montré que la relation empirique générait une sous-estimation importante. La validation du flux de chaleur sensible a fourni des résultats comparables à ceux rencontrés dans la littérature et concernant la validation de modèles sur des bases de données incluant de nombreuses situations.

International audience; The airborne PolDER data acquired during the ReSeDA experiment were used to assess the accuracy of the narrowband to broadband conversion, commonly used to express the integrated value of albedo as a linear combination of waveband estimates. We focused on the estimation of shortwave, visible and near infrared apparent albedos from waveband hemispherical reflectances. The latter were computed using four classical linear BRDF kernel-driven models. The relative discrepancy over the estimates from the four models was about 10%. The narrowband to broadband conversion was performed considering several coefficient sets devoted to recent sensors. The validation against field measurements showed a relative accuracy of shortwave albedo about 9%, whereas it emphasized the quality of visible and near infrared albedos through an indirect procedure. However, the consequences of the theoretical uncertainties, made when calibrating the coefficient sets we used, remain unknown. Similarly, the calibration of coefficient sets devoted to the shortwave albedo estimation from PolDER data showed that it was not possible to discriminate the information provided by the blue, green and red PolDER channels. Indeed, the hemispherical reflectance data set we used was not accurate enough to address experimentally the validity of the spectral interpolation method. Finally, the calibration of coefficients showed that it might be possible to calculate shortwave albedo with a relative error of 8% using only red and near infrared channels.; Les données aéroportées PolDER acquises durant l'expérimentation ReSeDA ont été utilisées pour évaluer la précision de la méthode "narrowband to broadband conversion", communément utilisée pour exprimer la valeur intégrée de l'albedo comme une combinaison linéaire de mesures spectrales. Nous sommes intéressés à l'estimation de l'albedo apparent visible-proche infrarouge-courtes longueurs d'onde à partir de la réflectance hémisphérique. Cette dernière a été calculée en utilisant quatre modèles classiques de FDRB à noyaux. La dispersion relative entre les estimations des quatre modèles était de l'ordre de 10 %. Nous avons calculé la valeur intégrée de l'albedo en utilisant des jeux de coefficients proposés dans la littérature pour des capteurs récents. La validation a montré que l'albedo courtes longueurs d'onde PolDER avait une précision relative de l'ordre de 9 %, alors qu'une procédure indirecte a mis en évidence la qualité des albedo visible et proche infrarouge. Cependant, les estimations de réflectance hémisphérique n'étaient pas assez précises pour évaluer les conséquences des incertitudes théoriques des jeux de coefficients. De même, la calibration de jeux de coefficients à l'aide des données ReSeDA a montré qu'il n'était pas possible de discriminer l'information fournie par les canaux PolDER bleu, vert et rouge. Finalement, il était possible de calculer l'albedo avec une précision relative de 8 % en ne considérant que les canaux rouge et proche infrarouge.